Construction materials constitute a major cost component in any construction project. The total cost of installed materials (or value of the materials) may be 50% or more of the total cost. Cost estimating is an assessment of the expected cost of any construction project. The accuracy of such an estimate has a serious effect on the expected profit of the construction contractor. Hence, a certain contingency premium should be added to the base estimate to increase the level of confidence. Such premium is materially affected by many factors. Through this research, the main factors that are expected to affect the accuracy of the construction materials' cost estimate were clearly identified. The numbers of these factors 46 were collected from the literature review. The twelve factors are identified as the most important factors. These factors were: Unsettlement of the local currency in relation to dollar value, fluctuation of price of raw materials, project materials monopoly by some suppliers, fraudulent activities of subcontractors, standard procedure for updating cost information, poor contractor experience for project type, improper planning and errors during construction, engaging in adequate skill on labor, waste control during material usage, location of project, improper construction methods and improper supervision at site and control